Derivative of a vector times constant

Question

I have a function $f: \mathbb{R}^n \to \mathbb{R}^n$ such that $f(\mathbf{x}) = \alpha \mathbf{x}$. Is the derivative $\frac{\partial f}{\partial \mathbf{x}} = \alpha \mathbf{J}$ or $\frac{\partial f}{\partial \mathbf{x}} = \alpha \mathbf{I}$, where $\mathbf{J}$ is a matrix with all 1s and $\mathbf{I}$ is a standard $n \times n$ identity matrix? Thank you!